Preventing formation of prussian blue stain in color developed photographic prints



2,706,687 Patented Apr. 19, 1955 PREVENTING FORMATION OF PRUSSIAN BLUE STANIN sIN COLOR DEVELOPED PHOTOGRAPHIC PR T Herbert L. Rees and Robert B. Henrickson, Rochester,

N. Y., assignors to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey No Drawing. Application July 20, 1953, Serial No. 369,238

7 Claims. (CI. 95-88) This invention relates to color photography and particularly to a method and means for preventing the formation of a Prussian blue stain in photographic prints which have been color developed and treated with a ferricyanide bleach.

It is well known that in certain processes of color forming photographic development, in which colored images are formed upon development by the action of the oxidation product of a primary aromatic developing agent upon a color forming compound or coupler, it is sometimes desirable to remove the developed silver image without destroying the residual silver halide or the dye image formed in the layer. A solution of a ferricyanide such as an alkaline solution of potassium ferricyanide is frequently used for this purpose. This solution converts the silver image to silver ferrocyanide, which is then removed with a solvent such as hypo. A certain amount of the ferricyanide of the bleach is retained in the emulsion or in the paper stock in the case of emulsions coated on a paper support and during or after processing a blue stain may appear in the emulsion or on the support. This stain may occur during the processing cycle, particularly if the paper subsequently passes through any acid solution or it may become evident upon prolonged keeping under adverse conditions of heat and humidity.

This blue stain is evidently Prussion blue formed in the following way. Ferricyanide from the bleach bath which is retained in the emulsion or support is converted to ferrocyanide during the subsequent hypo treatment, according to the following scheme:

Any iron which may be present in subsequent solutions converts the ferrocyanide to Prussion blue according to the following well known reaction:

One source of iron in the processing solutions is the piping and processing tanks through which the solutions pass. A source of iron is also the wash water or water used to make up the various solutions.

It is therefore an object of the present invention to provide a method and means for preventing the formation of this undesirable Prussian blue stain in color developed photographic prints. A further object is to provide a fixing composition which will serve to prevent the formation of a Prussian blue stain in color developed prints. Other objects will appear from the following description of our invention.

These objects are accomplished by incorporating in the fixing bath or in a separate bath used prior to fixing, a water-soluble salt of zinc, cadmium or manganese.

Processes of color photography in which color developed images are formed in photographic emulsion layers are described in Mannes, Godowsky and Wilder U. S. Patent 2,252,718, granted August 19, 1941, and in Jelley and Vittum U. S. Patent 2,322,027, granted June 15, 1943, Example 7. In the former patent a reversal process using couplers incorporated in the developing solutions is described and in the latter patent a negative process using couplers incorporated in the emulsion layers is described. Couplers may also be incorporated in the processing solutions in a negativepositive process and couplers may be incorporated in the emulsion layer or layers in a reversal process. In any of these processes the silver images may be converted to silver ferrocyanide by treatment with a ferricyanide bath prior to fixing with hypo or other solvent.

In color development processes of these types, we propose to incorporate in the hypo solution or in a solution used after the ferricyanide bath but prior to fixing with hypo, a soluble salt of zinc, cadmium or manganese. Zinc sulfate, for example, may be incorporated in the acid fixing bath and this compound reacts with the ferrocyanide in the paper to form insoluble white zinc ferrocyanide according to the following scheme:

NatFeCys +ZnSO4- ZnzFeCys This reaction proceeds to completion and removes the ferrocyanide so that none is left to form Prussian blue with the iron. Formation of a white precipitate is, of course, necessary since any colored precipitate would show in the finished print. Other salts which give a white precipitate with ferrocyanide such as soluble salts of cadmium and manganese can also be used.

The usual fixing bath contains a thiosulfate such as sodium, potassium or ammonium thiosulfate, an alkali metal sulfite such as sodium or potassium sulfite, acid such as acetic, citric or boric acid, and may contain other ingredients such as hardening agents, etc. A typical fixing bath would have the following composition:

Water to 1 liter.

The amount of zinc sulfate used in the acid fixing bath may vary from 5 to 10 or more grams per liter but usually these amounts are found to be adequate.

Our agents may also be used in dry packaged photographic compositions adapted to form an acid fixing bath upon mixing with water. Such compositions may contain about 50 parts of sodium thiosulfate, about 3 parts of desiccated sodium sulfite and about 1 part of zinc sulfate. Other agents such as hardening agents may also be present.

It will be understood that the examples and modifications included herein are illustrative only and that the invention is to be taken as limited only by the scope of the appended claims.

We claim:

1. The method of preventing the formation of ferric ferrocyanide in a color-developed and fixed silver halide emulsion layer which has been subjected to a ferricyanide bleach bath, which comprises fixing said layer after development and bleaching with said bleach bath, with a solution containing a fixing agent for silver halide, in the presence of a water-soluble salt of a metal selected from the class consisting of zinc, cadmium and manganese.

2. The method of preventing the formation of ferric ferrocyanide in a color-developed and fixed silver halide emulsion layer which has been subjected to a ferricyanide bleach bath, which comprises fixing said layer after development and bleaching with said bleach bath, with a solution of thiosulfate, an alkali metal sulfite and an acid, in the presence of a water-soluble salt of a metal selected from the class consisting of zinc, cadmium and manganese.

3. The method of preventing the formation of ferric ferrocyanide in a color-developed and fixed silver halide emulsion layer which has been subjected to a ferricyanide bleach bath, which comprises fixing said layer after development and bleaching with said bleach bath, with a solution of sodium thiosulfate, sodium sulfite, acetic acid and zinc sulfate. 7

4. A fixing composition adapted to fix a developed silver halide emulsion layer and prevent formation of colored ferrocyanides after treatment of the layer with a ferricyanide bleach, comprising a thiosulfate, an alkali metal sulfite, an acid, and a water-soluble salt of a metal selected from the class consisting of zinc, cadmium and manganese.

5. A fixing composition adapted to fix a developed silver halide emulsion layer and prevent formation of colored ferrocyanides after treatment of the layer with a ferricyanide bleach, comprising an alkali metal thiosulfate, an alkli metal sulfite, acetic acid, and zinc sulfate.

6. A fixing composition adapted to fix a developed silver halide emulsion layer and prevent formation of 10 colored ferrocyanides after treatment of the layer with a f erricyanide bleach, comprising sodium thiosulfate, SOdlUITl sulfite, acetlc acid and zinc sulfate.

7. A dry packaged photographic composition adapted to form an acid fixing bath suitable for fixing a colordeveloped photographic layer and preventing the formation of colored ferrocyanides in said layer after treatment of said layer with a ferricyanide bleach bath, comprising approximately 1 part of zinc sulfate, approximately 3 parts of sodium sulfite and approximately 50 parts of sodium thiosulfate.

References Cited in the file of this patent UNITED STATES PATENTS 1,624,172 Levy Apr. 12, 1927 

1. THE METHOD OF PREVENTING THE FORMATION OF FERRIC FERROCYANIDE IN A COLOR-DEVELOPED AND FIXED SILVER HALIDE EMULSION LAYER WHICH HAS BEEN SUBJECTED TO A FERRICYANIDE BLEACH BATH, WHICH COMPRISES FIXING SAID LAYER AFTER DEVELOPMENT AND BLEACHING WITH SAID BLEACH BATH, WITH A SOLUTION CONTANING A FIXING AGENT FOR SILVER HAL IDE, IN THE PRESENCE OF A WATER-SOLUBLE SALT OF A MELT SELECTED FROM THE CLASS CONSISTING OF ZINC, CADMIUM AND MANGANESE. 